Orthodontics is the specialty branch of dentistry dealing with the treatment of malpositioned teeth and the correction of improper relationships between the upper and lower dental arches. Generally, the practice of orthodontic treatment is effected by the application of mechanical forces to selected teeth or to an entire dental arch when malocclusion involving a full arch needs to be corrected. Numerous intraoral appliances are available for mounting directly on the patient's teeth with resilient wires or elastic bands used to provide corrective forces for moving malpositioned teeth into correct alignment.
An orthodontic headgear is an extraoral harness which includes a strap or arrangement of several straps configured to fit around the back of a patient's neck and/or around a patient's neck and over the patient's head and which is releasably connectable to a corrective extraoral tractive apparatus (face bow, set of J-hooks, chin cup, or the like) which is designed to apply a traction or posterior force to the patient's dental arch or teeth. Headgears have typically employed elastic bands or straps made of rubber, surgical latex or similar acting elastomers (in loop or multiple loop configuration) which are elongated and coupled to the teeth (usually through a wire face bow) to deliver the desired traction forces.
The term headgear as used in this application refers to a strap (or band) or series of straps (or bands) configured for either cervical mounting (single strap or band extending around the neck) or occipital (or calvarial) mounting (multiple straps or bands, one providing cervical anchorage and one or more providing occipital anchorage).
Commonly utilized types of orthodontic headgears present problems during orthodontic treatment. Principal difficulties arise with regard to the rubber, latex or elastomer bands normally used to provide traction forces. Such bands are manufactured with a guaranteed force predictability within the range of about .+-.15-25% of rated pull force at a given elongation. Thus, a new latex type force band or loop rated at 8 ounces pull at an elongation of 3 times its slack diameter might, in fact, provide as little as 6 or as high as 10 ounces of pull force at such elongation. Further, such bands have a short life and are elastically unstable, and the restoring force delivered by an elastic band for a given elongation (initial pull rating) drops off quite rapidly with time with the result that the patient must be instructed to replace the bands at frequent and regular intervals. More importantly, with common types of headgears utilizing such bands, as the patient moves his or her head, the elongation of the bands changes with the result that the force actually applied or delivered by the appliance may vary by a factor of .+-.2 to 3 times the desired force level due to the rapid degree of force change (increase or decrease with elongation or relaxation) from the rated force at the point of elongation establishing such force.
An important factor in an effective orthodontic treatment program is the ability to apply a known, substantially constant corrective force in the direction desired. With conventional headgears employing elastic force bands this ability is difficult to achieve because the treatment program, to a great degree, relies on full patient cooperation with respect to timely replacement of the bands. Even if the patient is fully cooperative, the variation of actual pull force between bands of the same rating and the rapid decrease in restoring force of such bands with time negates the probability of delivery of the desired substantially constant corrective force. This can increase the time required to accomplish a correction, or limit the effectiveness of the correction.
More recently, orthodontic headgears have been proposed, designed and provided with various types of metalic springs to apply corrective forces to a patient's teeth or jaws. In U.S. Pat. No. 3,526,035 granted to M. M. Armstrong there is disclosed a headgear using two wire coil springs to apply corrective bilateral forces to a patient's teeth or jaws. Such springs provide restoring forces (depending on their elongation) which are very stable and predictable over long periods of use, but the restoring forces are rapidly cumulative (rapid increase with spring elongation) so that for changes in spring elongation non-constant restoring forces are developed. Further, when the springs are utilized in a manner in which the nominal force is generated by an extension which is a large fraction of the spring resting length, small changes in its length cause large changes in applied force loads. Thus, during use of the orthodontic headgear as disclosed by Armstrong, movement of the patient's head (up, down or sidewise) and movement of the mandibular arch with respect to the maxillary arch results in large percentage extensions and retractions of the respective coil springs with the result that the spring-generated tractive forces vary significantly from the desired (pre-set) corrective force level.
U.S. Pat. No. 3,686,757, granted to J. C. McVickers and E. A. Leatherman, discloses an orthodontic appliance including a harness carrying force-applying members adapted to apply constant predetermined forces to a face bow or chin cup. Each force-applying member includes a flat, non-cumulative force, type of spring which provides a constant pull force as it is extended (or unwound) and as it rewinds. While the force-applying springs of the McVickers et al patent constant forces to the corrective appliance at all times, including when the head of the wearer is moved either from side to side or up and down, the constant force of each force member is predetermined by the characteristics of the spring contained therein. Thus, their force level cannot be changed or adjusted except by substitution of a spring having characteristics providing the different desired level of constant force. Non-cumulative force springs are expensive, require special heat treating and means for backwinding or otherwide pre-stressing the spring, occupy greater volume than cumulative (non-constant) force springs, and require careful attention during design and manufacture to the material from which they are made, to the thickness and width dimensions of the flat spring material, and to edge treatment in order to attain reasonable performance and life.
The orthodontic appliance of this invention overcomes the problems of: large variability of applied corrective force from the desired (pre-set) force level for wire coil springs used in the Armstrong type headgears during normal movements of the patient's head and/or jaws; and non-adjustability of the force level of costly non-cumulative force springs of the type used in the McVickers et al headgears. Rather, relatively inexpensive clock-type springs which develop non-constant (cumulative) forces (during their winding and unwinding) are used extraorally in a manner that results in the application of a substantially constant pre-determined corrective force during the orthodontic treatment program. Even though with head or jaw movement the clock-type springs of the appliance unwind and/or wind, the resultant changes in force developed are small compared to the preset or desired force level due to the movements entailing only a small portion of the total available extension of the spring. Further, the clock-type springs of the appliance can be wound or unwound by the treating orthodontist to adjust the magnitude of the applied corrective force level thereby eliminating the need for changing springs to obtain desired variations in force levels during a treatment program or between patients.